During the production of oil, natural gas and other wellbore fluids from a subsurface environment, it is frequently necessary to control the rate at which such fluids are produced from a wellbore. For many years, in both onshore and offshore production operations, pressure control assemblies (also referred to as "chokes") have been used with a well's valving and piping arrangements to help control production rates.
In onshore operations, the choke is installed on the surface valves and piping of a wellbore (also collectively known as a "tree") and is readily accessible by oilfield crews for maintenance. During maintenance, the choke simply is isolated from the tree and then removed for maintenance or replacement by the crew as necessary. However, in offshore operations where the tree and choke assembly are located in a subsea environment, the choke is not so readily accessible for removal by an oilfield crew. Accordingly, it is difficult to access and maintain or replace a subsea choke.
In a subsea production environment, there are various tree configurations that incorporate a choke assembly. In the most typical subsea arrangement, the choke body is incorporated into the tree piping. This arrangement requires the entire tree to be removed in order to retrieve and service the choke. In another design, the choke may have an insert design that allows the choke internals to be removed for maintenance. However, if the choke body is damaged, replacement of the choke body requires removal of the tree. A third choke design uses pre-installed connector assembly in which the complete choke assembly may be removed by a remotely operated vehicle without requiring the entire tree to be retrieved. Such a system is described in U.S. Pat. No. 4,589,493, issued to T. P. Kelly et. al. May 20, 1986.
A fourth type of subsea choke arrangement is described in U.S. Pat. No. 4,848,473, issued to G. E. Lochte July 18, 1989. In this patent, the choke and associated piping is mounted on top of the subsea tree in a piece of piping called a tree cap. The advantages of this design is that the entire choke assembly may be retrieved, repaired, and reinstalled without removing the entire tree from the subsea environment.
The Lochte patent includes a tree cap having three flow passages completely through the tree cap. These passages include a production stream conduit, a production return conduit, and an annulus connection conduit. Although the Lochte application is a significant improvement over previously existing subsea choke arrangements, the Lochte design has additional piping and valving requirements that are avoided through the improved tree cap and piping arrangement designs disclosed herein. Also, the Lochte embodiment requires three flow passages through the tree cap, as opposed to only two passages needed in the tree cap of the design disclosed herein. Also, the Lotche embodiment requires use of a workover riser embodying three vertical conduits, as opposed to only two conduits needed in the improved design disclosed herein. Additionally, in one of the embodiments described herein, it is possible to continue producing the well even after the choke experiences a failure, by isolating the failed choke from the main production flowstream. This improved design offers a significant advantage in the use of the described subsea choke system.